fate of fine sediment from dredger-based mining in a wave-dominated environment at chameis bay,...

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Fate of Fine Sediment from Dredger-Based Mining in a Wave-DominatedEnvironment at Chameis Bay, NamibiaAuthor(s): Geoffrey G. Smith, Nadia Weitz, Christoph Soltau, Anel Viljoen, Stephen Luger, and LimaMaartensSource: Journal of Coastal Research, Number 241:232-247. 2008.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/06-0677.1URL: http://www.bioone.org/doi/full/10.2112/06-0677.1

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Journal of Coastal Research 24 1 232–247 West Palm Beach, Florida January 2008

Fate of Fine Sediment from Dredger-Based Mining in aWave-Dominated Environment at Chameis Bay, NamibiaGeoffrey G. Smith†, Nadia Weitz†, Christoph Soltau†, Anel Viljoen†, Stephen Luger‡, and Lima Maartens§

†Council for Scientific and IndustrialResearch

P.O. Box 320StellenboschSouth [email protected]

‡Prestedge Retief Dresner Wijnberg (Pty)Ltd.

P.O. Box 52003WaterfrontCape Town, 8002South [email protected]

§De Beers Marine NamibiaP.O. Box [email protected]

ABSTRACT

SMITH, G.G.; WEITZ, N.; SOLTAU, C.; VILJOEN, A.; LUGER, S., and MAARTENS, L., 2008. Fate of fine sedimentfrom dredger-based mining in a wave-dominated environment at Chameis Bay, Namibia. Journal of Coastal Research,24(1), 232–247. West Palm Beach (Florida), ISSN 0749-0208.

Nearshore turbidity due to suspended fine sediment is a natural phenomenon around much of the western coast ofsouthern Africa. However, there is concern about the potential impact on biotic communities of an increase in turbidityas a result of mining by means of marine dredgers. This paper discusses field measurements and the validation andapplication of mathematical models in investigations of the structure, composition, and decay of fine sediment plumesinduced by dredging on a wave-dominated coast. In addition, the resuspension and mobility of the fine sedimentdeposited as a result of dredging are assessed. The expected trends of horizontal decay of concentrations with distancefrom the dredger and higher concentrations at greater depths were confirmed. The extent of plumes (i.e., whereconcentrations return to background levels) was predicted to be some 4 km at most, while a plume life span well inexcess of 3 h was indicated. This extent and longevity of suspended sediment plumes were attributed to the highmobility of fine sediment in a wave-dominated environment resulting from elevated bed shear stress induced by swellwave action. Predictions indicate that these highly mobile fine sediments are transported tens of kilometres withinweeks after discharge.

ADDITIONAL INDEX WORDS: Fine sediment plumes, dredging, circulation modelling, environmental monitoring,Chameis Bay, Namibia.

INTRODUCTION

Nearshore turbidity due to suspended fine sediment is anatural phenomenon around much of the western coast ofsouthern Africa. However, there are sensitivities with regardto potential environmental impact of diamond mining–relat-ed sediment discharges. A particular concern is the potentialimpact on the rock lobster resource in the region. Recently,the use of marine dredgers in mining operations has beeninvestigated by mining companies, leading to renewed con-cern.

The topic of dredging-induced plumes has been fairly wellresearched. HITCHCOCK and BELL (2004) showed that theform and magnitude of plumes are governed by dredgingtechnique (type of plant, method of overboard returns, speedof dredger), sensitivity of dredged material to resuspension(which depends on material characteristics), and the ‘‘condi-tion’’ of overlying waters (depth, current velocity and shear,turbulence, temperature, wave climate, salinity, etc.).

The available information indicates that dredger-induced

DOI:10.2112/06-0677.1 received 31 March 2006; accepted in revision18 January 2007.

Funding was provided by De Beers Marine Namibia (PTY) Ltd.and the Benguela Current Large Marine Ecosystem programme(Project BEHP/CEA/03/03).

plumes are relatively restricted in extent, in terms of mea-sured sediment concentrations. NEWELL et al. (1999) indicat-ed that the finest silt-sized particles ‘‘largely disappeared’’from the water column 480 m downstream of a Trailing Suc-tion Hopper Dredger (TSHD) while it was dredging marineaggregates. This corroborated their observations, from anumber of studies, that the settlement of inorganic particu-late load discharged from marine aggregates dredging is con-fined to a distance of a few hundred metres from the source.HEALY et al. (1999) measured concentrations close to back-ground levels �250 m from a backhoe dredging barge duringa small harbour dredging operation (dredging 2500 m3 of siltin a period of 5 wk). Trenching for a pipeline crossing anestuary resulted in suspended sediments, which settled to thebed ‘‘within a mile’’ of the dredging (ONUSCHAK, 1982). Forcases of larger-scale dredging, the distance for decay in con-centrations is greater. For example, measurements from RID-LEY THOMAS et al. (1998) indicate that from dumping 9000m3 of mud from a TSHD in 40 metres of water, near-back-ground concentrations were attained at a distance of 2 kmdowndrift. However, exceptions to this relatively rapid spa-tial decay of concentrations to a background level do occur.For example, HITCHCOCK and BELL (2004) found that a near-bed plume that was 2–4 m thick extended well beyond 4.5

233Fate of Fine Sediment from Dredger-Based Mining, Namibia

Journal of Coastal Research, Vol. 24, No. 1, 2008

Figure 1. Location diagram.

km downstream of dredging operations, and they also madereference to another plume measured 8 km from a dredgingsite in the North Sea.

While most of the aforementioned examples allude to thehorizontal decay of plumes with distance from the source,vertical changes are also generally found. Overall, concentra-tions increase with depth, primarily as a result of settlementof material and accumulation of this material just above theseabed. Several measurements of the effects of scallop dredg-ing indicate a near-linear increase in concentrations withdepth (BLACK and PARRY, 1999). Plume monitoring near acutter suction dredger in a harbour also indicated increasingconcentrations with depth (CARTER and COLES, 1998). How-ever, this is not always the case. MIKKELSEN and PEJRUP

(2000) indicated some higher concentrations near the surfacerelative to mid-depth in a plume induced by dredging. Datadepicted in BOHLEN et al. (1979) also indicate somewhat var-iable concentration distributions close to the dredger, withmid-depth concentrations at times higher than bottom con-centrations. These inverted trends seem to occur close to thedredger sediment source (e.g., up to roughly 250 m from thedredger in the BOHLEN et al. [1979] case) and revert to amore typical trend of increasing concentrations with depthfurther from the dredger.

Plumes are generally relatively short-lived after dredgingceases. During scallop dredging at Portarlington (where a bedmud content of 30.1% is indicated), 98% of the sediment inthe plume was observed to fall back to the seabed within 30min after passage of the dredger (BLACK and PARRY, 1999).A plume from a backhoe dredging operation was estimatedto last only 5–15 min after its production at the dredger(HEALY et al., 1999). Sediment concentrations from disposalof fine sediment by large dredgers (2800 m3 to 9200 m3 hop-per capacity) were shown to decrease to near-background lev-els within a few hours (RIDLEY THOMAS et al., 1998). How-ever, exceptions to these data may result from specific com-binations of sediment (e.g., very fine sediment experiencinglimited flocculation) and hydrodynamic and seabed condi-tions.

Against the background of these observations of plume be-haviour, the primary objective of the present study was toinvestigate the structure, composition, and decay of fine sed-iment plumes induced by a unique dredger-based mining op-eration on a wave-dominated coast by employing both a mea-surement and a computational modelling approach.

While dredger-induced plumes are indicated to generallyhave a constricted and short-lived ‘‘footprint’’ of elevated sus-pended sediment concentrations, subsequent resuspension,advection, and diffusion of deposited sediment is likely to ul-timately result in widespread distribution of the dischargedor disturbed sediment. For example, HITCHCOCK and BELL

(2004) quoted an example of sediments from a dredging op-eration being found 35 km from the source. If fines are well-dispersed, deposition will most likely occur in very thin layersover large areas. HEALY et al. (1999) determined thatdredged sediment does deposit in very thin layers, as wasintended for their small dredging project. Fine material islikely to be dispersed more widely in a wave-dominated en-vironment as a result of frequent resuspension of fine sedi-

ment deposited from dredging activity. Such resuspensionhas been found to occur in the event of significant swellwaves (JING and RIDD, 1996). Against the background of aswell wave–dominated environment, it was the second objec-tive of this study to investigate the resuspension and mobilityof fine sediment deposited as a result of the dredger miningoperation, both from measured data and by means of mod-elling.

Physical and Environmental Frame

Chameis Bay, located in Namibia, about 110 km north ofthe Orange River (Figure 1), is the region of a trial dredgingoperation to mine diamonds from the seabed. This is one ofmany mining operations within the Sperrgebiet (‘‘forbiddenterritory’’), which extends for some 300 km along the south-ern coast of Namibia. From 22 January to 5 February 2005,diamondiferous material was excavated by means of a TSHDwith a hopper capacity of 7850 m3 at two sites (northern andsouthern dredging sites, Figure 2). The dredged material waspumped into a pit on the upper beach, which was left overfrom previous opencast mining, for processing at a laterstage. The pit was protected from wave attack by a massive

234 Smith et al.


Figure 2. Site plan showing physical features and measurement loca-tions.

sand seawall (Figure 3). Dredged material was pumpedthrough a 900-mm-diameter pipeline, which extended fromthe pit to about 600 m offshore, where the water depth was16 m. During and subsequent to the dredging operation (from22 January to 28 February 2005), material mined by meansof conventional earthmoving equipment from the backbeachwas processed at a plant to separate out diamond-bearinggravels, resulting in the discharge of sediment finer than 1.4mm onto the intertidal beach (for location of this plant dis-charge, see Figures 2 and 3).

The southern dredging site was situated in about 25 mdepth, within Chameis Bay (Figure 2). This bay has a pre-dominantly sandy shoreline �8 km long. The bay is formedby the headland, Panther Head, and is interrupted by a rockyoutcrop (Figure 2). Sand transported and deposited by waveaction on the northern shore of the bay feeds a dune field(approximate location, see Figure 2). The northern dredgingsite is situated in 15 to 22 m water depth to the northwestof Chameis Head (Figure 2), a rocky headland separatingChameis Bay and the beach at the dredged spoil site to thenorth.

The dredging operation was conducted in summer, duringwhich winds tend to be strong: hindcast wind data (ROS-SOUW, 2002) for a location about 60 km offshore of the study

site indicate that southerly winds with speeds of over 6 m/soccur for 75% of the time in summer. Wind data measuredat the site confirm this windy summer climate: an averagewind speed of 6.8 m/s (standard deviation � 3.2 m/s) wasdetermined for the period 22 January to 28 February 2005;over 80% of winds were from south to southeast.

Wave heights are generally lower in summer than in otherseasons. Time series of offshore wave conditions during thestudy showed a mean wave height of 1.9 m (standard devia-tion � 0.5 m), with a persistent south-southwesterly directionof approach (average offshore wave angle � 201.5�, standarddeviation � 8.7�) as shown in Figure 4. The average waveperiod of 10.0 s (standard deviation � 2.1 s) reflects the in-fluence on waves of strong southerly winds in the South At-lantic, which dominate over more typical remotely generatedswell that has wave periods of 12 to 14 s. Refraction andshoaling of the average wave conditions result in reducedwave heights that approach the shoreline at a slightly obliqueangle, according to model simulations (Figure 5). These waveconditions, combined with the summer southerly wind con-ditions, generate northbound currents that tend to follow thecoastline, as indicated by a flow model simulation (Figure 6is a typical predicted flow field). From this simulation, peaksin current magnitude of 1.5 m/s to 2.0 m/s were predicted forthe near-beach region during the period 22 January to 28February 2005. These strong flows are wave-generated, whilethe weaker flows (0.2–0.3 m/s) further offshore are wind-gen-erated. Tides were found to have a limited influence on flow(variability in nearshore measured currents correlated withdiurnal variations in wind rather than tides). Reversals inflow direction occur during relatively infrequent northwest-erly winds (e.g., as occurred on 29 January; see Figure 4).

Available measurements of naturally occurring suspendedmatter in the region indicate that concentrations are gener-ally low but are higher within naturally occurring plumes.Twenty-eight water samples (during eight days) taken be-yond the limits of visible naturally occurring plumes indicat-ed an average suspended particulate concentration of 2.3mg/L, with a maximum of 7.0 mg/L. These results are wellwithin the range of values recorded on the South Africancoast, which was found to be 1.5–25.7 mg/L (ZOUTENDYK,1992), and also within the range of suspended inorganic par-ticulate concentrations (i.e., excluding organic material) mea-sured on the west coast of South Africa and Namibia, whichwas found to be 1–14 mg/L (ZOUTENDYK, 1995). However,surface samples targeting observed naturally occurringplumes that were deemed to be unaffected by mining hadconcentrations of 14–26 mg/L. Profile measurements indicat-ed that particulate concentrations of this order of magnitudeextend from the surface to the seabed at times.

Dredger and Mine Sediment Input

During each dredging operation, material is sucked upfrom the seabed at a high rate. This material is dischargedinto the dredger hopper and allowed to settle. Surplus over-spill water is drained off and discharged near the base of thedredger (at about 9 m below the surface when fully loaded).This overspill water contains sediment in suspension. The



Figure 3. Aerial view of the dredged spoil site. Plumes resulting from mine tailings discharge are evident.

horizontal location of this dredger discharge moves consid-erably and rapidly as the designated site is dredged (Figure7).

The amount of sediment released from overspill was esti-mated from the product of the volume of overspill (as record-ed by the dredging company) and the total suspended solidsconcentration of the overspill (as sampled during the dredg-ing project). An alternative estimate from swath surveysbased on the difference between the total volume of sedimentremoved by the dredger and the volume of sediment removedfrom the seabed was found to be excessively sensitive to bothsurvey accuracy and the accuracy of estimated hopper vol-umes (the latter being affected by bulking factors). The for-mer, more reliable method, gave a total overspill dischargeof 4100 tons of fine sediment discharged from the dredgingof the southern site, while a total of 7400 tons was estimatedfrom dredging of the northern site. The times of overspill dis-charge and the average estimated rate of sediment dis-charged per dredging operation are depicted in Figure 4 formining of the southern dredging site from 22 to 28 January2005.

Sieving and settling tube analysis of 16 samples from thetotal of 33 dredger trips/operations indicated that an averageof 15% of the material was fine sand (D50 � 107 �m) and 85%

of the material was finer than 63 �m. In addition, 20% of thematerial was indicated to be clay (�4 �m). Sediment with afraction of clay particles higher than 10% is expected to havecohesive properties (VAN RIJN, 1989) that will result in theformation of flocs in suspension, which will settle to the sea-bed more rapidly than individual particles. In order to assessthis potentially accelerated settling of flocculated material,the laboratory-based pipette method (OWEN, 1971) was em-ployed to assess the settling velocity of the original sampledoverspill material. Since settling of particulates can be sen-sitive to the concentration of material in the water column,the test was conducted at two concentrations (4 g/L and 10g/L). An increase in settling velocity was found for the ma-terial with the higher concentration, as expected for sedimentconcentrations in saltwater that range from 0.1 to 10 mg/L(VAN RIJN, 1989). The results facilitated selection of the set-tling velocity (ws) of three fractions of sediment required formodelling as indicated in Table 1. Limitations of the pipettemethod include the inability to represent the effects of nat-urally varying temperature and natural turbulence-inducedshearing rates (which affect floc size) in the laboratory (MIL-LIGAN, 1995; OWEN, 1971). Another potential source of erroris the change in the nature of the sample due to organic pro-cesses (e.g., bacterial activity) between the time of sampling

236 Smith et al.


Figure 4. Time series of wind speed and direction measured at the beachwind station (Figure 2), wave height (Hmo), wave period (Tp), and meandirection, average overspill rate per dredging trip (indicated at the timeof the overspill), middepth suspended solid concentration at Site A (Fig-ure 2), and corresponding bed shear stress calculated from wave and flowmeasurements at Site A.

Figure 5. Predicted wave field for the offshore condition Hs � 2.03 m,Tp � 12.5 s, and mean direction � 206�.

and laboratory testing (MILLIGAN, 1995). These limitationsin determining settling velocity in the laboratory are recog-nised and must be considered when interpreting model pre-dictions.

In addition to dredger overspill sediment, material origi-nates from another mining source. After screening and sep-arating diamondiferous material mined in the backbeach re-gion, sand, silt, and clay finer than 1.4 mm was dischargedfrom the mine processing plant into the intertidal zone of thebeach. Between 22 January and 28 February 2005, an aver-age of 3822 tons (standard deviation � 1080 tons) was dis-charged to the beach daily. Sampling of this discharge indi-cated that about 86% of this material was coarser than 63�m, and the remaining 14% was fine. The discharge of thesand fraction caused considerable beach accretion, which af-fected the transport and dispersion of all discharged material.The fine material discharged during the study period totalled20,800 tons. The fine fraction (�63 �m) of this material had

a clay content of 41%, much higher than that of the dredgedmaterial. This fine content is reflected in the modelled frac-tions of fine sediment (Table 1).

METHODS

Environmental Measurements

A Mike Cotton Systems wind station was set up in an un-obstructed area on the beach in Chameis Bay (location inFigure 2). The base of the station was about 3 m above sealevel, and the wind sensors were mounted 5 m above theground on a slender mast. Hourly average wind speed anddirection were recorded. Currents and wave conditions weremeasured at two sites, one at Site A in 25 m depth and oneat Site B in 10 m depth (Figure 2). At Site A, currents andwaves were measured at middepth by means of a SEAPACSP2100. Waves were recorded by the SEAPAC instrument for1024 s every 3 h at 2 Hz, while currents were measured for1024 s at 2 Hz, and averages were recorded every hour. AnAanderaa RCM9 current meter was also located at middepthat this site, where currents measured at 1 Hz were repeatedlyaveraged over 10 min and recorded. At Site B, currents andwaves were measured by a single SEAPAC SP2100 situatedat middepth, with measurements conducted in the same wayas for Site A. In addition, currents were intermittently mea-



Figure 6. Predicted flow field for typical summer conditions (Hmo � 2.03m, Tp � 12.5 s, wave direction � 206�, wind speed � 11.84 m/s, winddirection � 138�).

Figure 7. Track of the dredger drag-head 1335 to 1535 on 28 January2005.

sured by tracking drogues, representative of flows at the sur-face, at 5 m depth, and at 10 m depth.

In addition to these measurements, offshore directionalwave measurements, which provided significant wave height,wave period of peak energy, and mean wave direction, wereobtained from a TriAxis Buoy situated about 80 km to thesoutheast.

Water Sampling and Analysis

A total of 119 water samples was taken during eight daysof the dredging operation. At each sampling site, a 2 L bottlewas filled with water from a depth of �30 cm below the sur-face.

The total suspended solids (TSS) concentration of eachsample was determined through filtration of a known volumeof a sample through glass-fibre filter paper (pore size 0.7 �m)of known mass. The filter paper was then dried at 105�C andreweighed. The mass trapped on the filter paper was the sus-pended material, which, when divided by the filtered volume,yielded the TSS concentration.

Selected samples were further analysed to determine theorganic and inorganic particulate content. The organic frac-tion was determined as the percentage of the sample that was

lost on ignition at 450�C in an autoclave. The mass remainingwas taken to be the inorganic fraction of the sample.

Turbidity

Turbidity was measured by means of an optical backscattersensor (OBS; DOWNING et al., 1981) deployed from a boat tomeasure turbidity profiles. In order to translate the OBS tur-bidity units to sediment concentration, calibration of the in-strument was conducted in the laboratory by recording theOBS turbidity units for known concentrations of the dredgeroverspill material that had been stirred up in a large testvessel. A linear relationship (number of experimental obser-vations � 10, R2 � 0.967) was determined from this data.

238 Smith et al.


Table 1. Settling velocity (ws) characteristics of modelled sediment.

Material

% Materialwith ws

� 1.4 mm/s

% Materialwith ws

� 0.08 mm/s

% Materialwith ws

� 0.01 mm/s

Dredger overspill 43 29 28Plant discharge 11 47 42

Figure 8. Orthogonal, curvilinear grid as applied for both wave and cur-rent modelling. Left frame shows the extent of the grid. Refinement inthe nearshore area is shown on the right.

This relationship was employed to calculate concentrationsmeasured in the field.

A turbidity sensor operating on a similar principle is anintegral part of the Aanderaa RCM9 instrument. Turbidityat site A measured at 1 Hz, was repeatedly averaged over 10minutes and recorded. The sensor was also calibrated in thelaboratory. The resulting linear relationship (number of ob-servations � 8, R2 � 0.875) was also employed to determinethe middepth suspended sediment concentration time series(Figure 4). Turbidity recordings were affected by marinegrowth on the sensor. It is difficult to discern exactly whenthis effect commenced. However, it is likely that the effectcommenced some time after 11 February and became so se-vere that no meaningful measurements were obtained after22 February.

It must be acknowledged that matter other than sediment(e.g., seaweed particles, air bubbles, detritus, or even fish)may have been misinterpreted as sediment. In addition, someof the sediment may have had a different grain size to thatused for the calibration. As the OBS is an order of magnitudemore sensitive to fine cohesive sediment than to sand (LUD-WIG and HANES, 1990), small amounts of sand in suspensionshould not be problematic. Nevertheless, the need to criticallyinterpret OBS measurements is noted.

Sediment Deposition Potential

Two sediment traps were deployed to collect material insuspension in the water column. One sediment trap was de-ployed at Site A (Figure 2), �2.5 m above the seabed, whilethe second trap was deployed at the same elevation above theseabed and in the same depth of water at a control site sit-uated �6.2 km south of the southern dredging site. Each trapconsisted of four individual vertical cylinder traps held byhorizontal cross-members to a vertical mooring line. The cyl-inders were open at the top, with baffles and a 200 �m filterin the upper quarter, and closed at the bottom. Each cylinderhad a trap volume of �2 L, with an aspect ratio (height/width) of 7. Tall traps have better trapping efficiency thanshort, wide traps (GARDNER, 1980) because eddies within thetrap are reduced. According to BACON (1996), the absoluteaccuracy of sediment trap measurements, particularly in hy-drodynamically active zones, is not certain. Nevertheless,through measurement of the downward flux of suspendedparticles, these traps provide an indication of the maximumpotential deposition that would occur on the seabed if con-ditions were such that no resuspension of settled sedimentwas induced. While this is a hypothetical situation, consid-ering the rapid mobilisation of fine sediment in the wave-dominated environment, it is nevertheless useful to consider

the maximum deposition that could have occurred at the con-trol and near-dredging sites.

Computational Modelling

Wave Model

The SWAN wave generation and refraction model was used(BOOIJ et al., 1999) within the DELFT3D-WAVE software(WL � DELFT HYDRAULICS, 2002). The processes simulated inthe model were: wave refraction, shoaling, bottom friction,and breaking. Local wind-wave generation was not modelledbecause it had been previously found to have only a minorinfluence on nearshore conditions in the area (SMITH andSOLTAU, 2002). Larger-scale wind effects on waves were ac-counted for in the wave measurements used as input to themodel. The coefficients applied in the model have been usedin a number of studies on the southern African west coastand were deemed to be applicable for this study. A JON-SWAP spectral shape was applied with a peak enhancementfactor (gamma) of 2.0. The bottom friction formulation ofMADSEN et al. (1988) was used. Model bathymetry was ob-tained from hydrographic charts as well as recent detailedmeasurements commissioned by the mining company. An or-thogonal, curvilinear grid was applied (Figure 8). The gridresolution ranged from approximately 6300 m � 3500 m atthe boundaries of the model to 90 m � 20 m at areas wheredetail was required. This grid covered a distance of �110 kmlongshore and 50 km cross-shore.

Comparisons of predicted wave heights with measuredwave heights at Site A and at Site B are shown in Figure 9.Reasonable agreement between measured and simulated con-ditions was attained after the bottom friction coefficient hadbeen increased from the default value of 0.05 to a value of0.08.

Hydrodynamics

Simulation of currents and turbulence was achieved bymeans of the DELFT3D-FLOW model, the hydrodynamic



Figure 9. Comparison of predicted wave height (solid line) with mea-sured wave height (dotted line) time series.

Table 2. Main input parameters for the hydrodynamic model.

Parameter Value/Details

Time step 1 minWind drag coefficients 0 m/s: 0.0011

100 m/s: 0.0065Bed friction formulation White-ColebrookNikuradse roughness length 0.03 mHorizontal eddy viscosity 5 m2/sBackground vertical eddy viscosity 0 m2/sTurbulence model k-LCorrection of sigma coordinates On

Figure 10. Comparison of predicted flow (solid line) with measured flow(dotted line), middepth at Site A. North/south component of flow is indi-cated above (north is positive); east/west component of flow is depictedbelow (east is positive).

module of the DELFT3D modelling suite (WL � DELFT HY-DRAULICS, 2003a). In this model, the time-dependant shal-low-water equations are solved in three dimensions in orderto simulate tide- and wind-driven flows. Drogue measure-ments from the water surface, at 5 m depth, and at 10 mdepth indicated that the three-dimensional approach wasnecessary. Measurements on both 24 and 29 January indi-cated that flows at 5 m and at 10 m depth were approxi-mately at right angles to the surface flow. Large variationsin measured velocity (e.g., ranging from 61 cm/s at the surfaceto 18 cm/s at 5 m depth to 4 cm/s at 10 m depth on 24 Jan-uary) also indicated strong vertical shear. The model takesaccount of tidal forcing, the Coriolis force, density-drivenflows, space- and time-varying wind and atmospheric pres-sure, bed shear stress at the seabed, and turbulence-inducedmass and momentum fluxes.

The main input parameters employed in the model are in-dicated in Table 2. Wind drag coefficients were obtained fromHSU (1988). The DELFT3D-FLOW default Nikuradse rough-ness length was employed (WL � DELFT HYDRAULICS, 2003a).A typical horizontal eddy viscosity based on the currentspeeds and grid size for the model was used. Zero backgroundeddy viscosity above that predicted by the k-L model (selectedfor expedience rather than the k-� model) was assumed. Cor-rection of sigma coordinates was applied in order to avoidartificial numerical creep.

The same orthogonal, curvilinear grid was applied for thehydrodynamic model simulations in the horizontal dimensionas was applied for the wave modelling (Figure 8). In the ver-tical dimension, layers were employed with thicknesses of10%, 20%, 40%, 20%, and 10% of the local water depth, wherethe thinnest layers were located both at the sea surface andnear the bottom in order to represent the boundary layers.The bathymetry employed was the same as for the wave mod-el mentioned already.

At the open boundaries of the model (i.e., the southern,western, and northern boundaries), only sea levels were spec-

ified. Although tides on the west coast of Africa have a lowtidal phase lag and therefore do not generate strong tidalcurrents along open shorelines, some tidal currents are likelywithin the semi-enclosed Chameis Bay. Tides were thereforeincluded by means of water-level variations applied to themodel boundaries, based on the tidal constituents from twotidal stations: Luderitz Bay (150 km to the north) and PortNolloth (186 km to the south). Wind data based on measure-ments at the wind station were applied in the model simu-lations. The rate of energy dissipation associated with waves,calculated by the SWAN model, was used as a source termin the DELFT3D-FLOW model to simulate wave-driven cur-rents. Wave-current interactions were not simulated becausethe effect of currents on waves was deemed to be negligible.

The integrity of the model is manifested by correlation be-tween measured and predicted flows. The north/south andeast/west components of the current velocities (tidal- andwind-driven) predicted mid-depth at Site A (Figure 2) werecompared against the measured current velocity components(Figure 10). In this figure, northward and eastward flows areplotted as positive values. The general trends of flow vari-

240 Smith et al.


Table 3. Main input parameters for the suspended sediment transportmodel.

Parameter

Value

Fraction 1* Fraction 2 Fraction 3

Critical shear stress fordeposition (Pa)

0.02 0.05 0.10

Critical shear stress forresuspension (Pa)

0.2 0.2 0.2

Zero-order resuspension flux(g m2 s1)

0.3 0.3 0.3

Sediment transportformulations

Sedimentation/erosion: Partheniades-Krone(Krone, 1962; Partheniades, 1962)

* Fraction 1, Fraction 2, and Fraction 3 refer to the three size fractionsof discharged material, i.e., the percentages as indicated in Table 1.

ability compare reasonably well, though the representationof north-south velocities during periods of low flow is lessaccurately represented. In the east/west direction, the mod-elled current velocities appear to be mainly westward, al-though the measured current velocities indicate instances ofeastward movement. Nevertheless, since the dominant flowswere reasonably well represented, the modelled results wereconsidered adequate for the purpose of this study.

A comparison between predicted and measured currentsmiddepth at Site B (Figure 2) in 10 m total depth showedlimited agreement. Close inspection of the measurements in-dicated that they were poorly correlated to wind and waveconditions. Unless caused by a local topographical featurethat was not resolved in the model bathymetry (some rockoutcrops were observed during the field measurements), thelack of correlation of measured flows with environmentalforcing was deemed to be the result of a flow instrumentationor deployment error.

Suspended Sediment Transport Model

The DELFT3D-WAQ water-quality model (WL � DELFT HY-DRAULICS, 2003b) was used to simulate the advection, dis-persion, settling, deposition, and resuspension behaviour ofthe dredging- and plant-induced suspended sediment. In themodel, the advection-dispersion equation was solved withsource and sink terms representing sedimentation and re-suspension. The DELFT3D-WAQ model obtained the hydro-dynamic input through an offline coupling to DELFT3D-FLOW and employed the same curvilinear grid. The settlingcharacteristics for three representative fractions of sedimentwere applied as per Table 1.

As the sediment particles approach the seabed, the proba-bility of deposition is determined by the prevailing seabedshear stress (seabed) relative to the defined critical shearstress for sedimentation of the particles (sed). Deposition willoccur only if seabed � sed. The total bottom shear stress con-dition is the sum of the wave- and the current-induced com-ponents and varies in both space and time. The resuspensionflux is specified by the critical shear stress for resuspension(res) and a zero-order resuspension flux (Zres). Resuspensionwill occur only if seabed � res. These parameters, as indicatedin Table 3, were derived from typical values in literature (e.g.,VAN RIJN, 1987).

In order to compute the thickness of fine sediment depos-ited on the seabed in the model, a typical consolidation con-dition after one month was assumed. This was characterisedby a porosity of 90%, a wet density of 1188 kg/m3, and a drydensity of 2650 kg/m3 (VAN RIJN, 1993).

A limitation of the model is that the dredger overspill dis-charge is represented by a single discharge into a model cell(with dimensions of roughly 50 m � 90 m � 8 m), while inreality, the dredger discharge was continuously moving withthe dredger. The rapid variability in the location of the dredg-er discharge (Figure 7) could have been represented with theappropriate preprocessing of data. However, as the focus ofthe study was not to assess near-field plume dynamics, thiswas not a concern. In terms of far-field plume dynamics, theprimary concern was exceedance of high concentrations (rath-er than persistence of concentrations just above backgroundlevels). Thus, the conservative approach of diluting the dis-charged sediment into a single cell only, which represents arelatively small volume around the dredger (where propeller-induced turbulence would promote initial dilution) was con-sidered to be appropriate.

Another limitation is that the sand fraction (indicated frommeasurements to be 15% of the discharge material) is ex-cluded from the plume modelling. This material is expectedto settle to the bed rapidly; based on the median grain sizeof the sand fraction (106 �m), settlement would occur at arate of about 6 to 7 mm/s. As a result of this settling velocity,the presence of sand would only apply to plumes occurringwithin an hour after dredging.

Validation of predicted sediment concentrations was re-stricted by the fact that (i) measured near-field suspendedsediment concentrations are by nature highly variable intime and space, making for poor comparison with the moretemporally and spatially averaged model results, while fur-ther afield, (ii) concentrations were not high enough to bedistinguished from background, naturally occurring concen-trations. Despite these limitations, it was found that, whilethe exact geographical extent of plumes was not totally cor-rectly predicted, measured and predicted concentrations gen-erally compared reasonably well (within a range of �20 mg/Lat most) on five out of eight days of available plume mea-surements. Poor correlation on the other three days may havebeen due to poor definition of the overspill rate as a result oflimited measurements of the overspill concentration at thesetimes.

Impact Thresholds

The modelling and measurement results of this study areultimately to be interpreted in terms of potential impacts onbiota. A review of thresholds applied on the west coast ofsouthern Africa (SMITH et al., 2006) indicated that at a sus-pended solids concentration of more than 100 mg/L (persist-ing for more than a few hours to days), impacts on some biotawould be expected. Thresholds of impact by deposited sedi-ment were found to be poorly defined. However, a review spe-cific to the west coast of southern Africa in SMITH et al. (2006)indicated that ‘‘submillimetre-thick’’ sediment deposition



Table 4. Comparison of daily averages of surface particulate concentra-tions from plumes and control sites.

Date

Number ofSamples

Control Plume

Daily Average(mg/L)

Control Plume

StandardDeviation

(mg/L)

Control Plume

Maximum(mg/L)

Control Plume

23/1/05 4 8 10.3 8.7 10.6 11.0 26.0 35.524/1/05 5 10 8.8 3.8 13.0 1.7 32.3 6.925/1/05 5 9 1.9 5.9 0.5 6.9 2.5 24.026/1/05 2 9 10.5 9.2 11.8 12.1 18.8 40.527/1/05 5 5 4.9 14.6 5.2 16.9 14.1 43.729/1/05 4 6 9.8 9.2 8.2 10.5 17.1 27.030/1/05 5 1 1.4 1.4 0.4 N/A 1.6 1.431/1/05 5 6 1.4 7.5 0.7 9.0 2.1 19.9

Figure 11. Suspended solids concentration profiles measured 60 m northof the southern dredging area during dredging operations.

lasting hours to days would probably result in a low or neu-tral impact.

RESULTS

The Structure and Composition of Plumes

Table 4 presents a comparison of surface concentrationsmeasured in plumes within 1.4 km of each of the sites beingdredged and control concentrations measured far from theinfluence of any dredging. Average daily concentration, stan-dard deviation of this data, and the daily maxima are pro-vided. It is interesting to note that less than half of the av-erage concentrations measured within visible plumes werehigher than for the control sites. However, on six out of eightdays, the maximum concentrations recorded in plumes werehigher than those for control sites. The standard deviationsindicate considerable variability in surface concentrationsboth at control sites and in plumes.

For the eight days of measurement, an overall average con-centration of 7.9 mg/L and a maximum concentration of 43.7mg/L were recorded in the plumes. These values are onlyslightly higher than the average and maximum natural con-trol measurements of 6.1 mg/L and 32.3 mg/L, respectively.

OBS measurements at control sites were not sufficientlysensitive to allow a similar comprehensive comparison of con-trol and plume concentrations at depth. However, it is evi-dent that plume concentrations were considerably higherthan control concentrations at depth. In addition, theseplume concentrations increased with depth. An average con-centration of 14.89 mg/L was determined at mid-depth for theeight days of measurements, with a near-bed average of 28.61mg/L. Maxima at middepth and near-bed were 61.7 mg/L and150.2 mg/L, respectively.

Plumes were generally found to manifest high spatial andtemporal variability. Figure 11 depicts a vertical profilethrough a plume, measured about 60 m north of the southerndredging area, during a dredging operation (which com-menced 25 min prior) on 24 January 2005. The profile indi-cates large variations in vertical structure. Even though theplume would have been recently created (dredging was inprogress at the time of measurement), a gradient in concen-trations is evident from the surface to the seabed, probablyas a result of settlement of material to the seabed. Elevated

concentrations close to the bed may partly have resulted fromthe fine sand fraction, which could have settled from the dis-charge position at 9 m depth to a region close to the bed fromthe time of commencement of dredging. The difference inplume structure between the ‘‘downcast’’ and ‘‘upcast’’ mea-surements is evidence of the rapid temporal variability inplume structure close to the dredger. Differences in concen-tration that occur at a depth of 0 to 15 m represent changesin the plume that occurred in a period of two to three min.These changes were probably partly induced by flows andturbulence induced by the dredger itself.

A sequence of profile measurements made in the dredgerplume also aids in understanding plume evolution. Theplume in Figure 12a was measured 25 min after the end ofdredging, on the southern edge of the southern dredging area,at 1710 on 23 January 2005. Prior to the measurement,winds had been southeasterly, changing to southerly just be-fore the measurement. These winds were the cause of flowswhich decreased with depth from 0.27 m/s (surface) to 0.12m/s (5 m depth) to 0.08 m/s (10 m depth) in a northerly tonorth-northwest direction, as measured by means of droguetracking at the northern end of the southern dredging region.

Given that discharge of fine sediment originally occurredat 9 m depth, it is clear from Figure 12a that there was anincrease in sediment concentration toward the seabed. Thiswas probably largely due to settlement of material to the bed.In addition, dilution of concentrations due to advection of finematerial (to the north) would have occurred. The decreasingdilution with depth (as a result of decreasing flows) wouldtend to promote an increase in concentrations. These effectswere seen to progress in subsequent plume measurements.The profile illustrated in Figure 12b was measured in thedredger plume 4 min after that of Figure 12a and 120 m to

242 Smith et al.


Figure 12. Vertical concentrations of suspended solids measured (a) 25min after dredging on the southern edge of the southern dredging area,(b) 4 min later, 120 m north-northwest of plume in a, and (c) 6 min fol-lowing and 290 m north-northwest of plume in b.

the north-northwest. By this time, it was evident that theconcentrations throughout most of the water-column had de-creased partly due to dilution through advection to the northand diffusion but also due to settlement: high concentrationsof settled material were evident near the bed, where shearstresses were sufficient to prevent deposition on the bed. Theprofile in Figure 12c was measured 6 min after measurementof the second plume profile (Figure 12b), at a location 290 mnorth-northwest of the second plume. At this time, the set-tlement of material was evident from increased concentra-tions restricted to depths near the bed, with correspondinglysubdued concentrations further above the bed.

Even when employing fine sediment discharge into a singlemodel cell, rapid variability in plume concentrations is pre-dicted by means of numerical modelling. Figure 13 shows asnapshot of predicted surface and bottom concentrations re-sulting from dredging at the southern dredging site as wellas plant tailings discharge. At both the surface and near thebed, a plume is evident at the northern end of the southerndredging site. This simulated plume is the result of dredgingcompleted �3 h earlier. Other simulated plumes furthernorth are remnants of earlier dredging operations. Theseplumes merge with the plume resulting from tailings dis-charge, resulting in a cumulative effect of the two sources.As was found from measurements, higher concentrations areevident at greater depths. For example, near the southerndredge site, an elliptical area, �800 m long and 400 m wide,of concentrations over 20 mg/L was observed near the bed,while lower concentrations (�10 mg/L) occurred at the sur-face over a smaller area. Higher concentrations were also ob-served further north at the bed relative to lower surface con-centrations.

The time series of sediment concentrations measured atmid depth at Site A (Figure 4) indicates a clear correlationwith dredging operations at the southern dredging site.Peaks in measured suspended solids concentrations (of up to20 mg/L) occur with every dredging event. After each event,the turbidity decreases to between 1 and 2 mg/L (i.e., close tothe background level), indicating the passage of individualplumes, as was predicted with the model. When the dredgermoved to the northern dredging site, suspended solids con-centrations decreased to background levels until subsequentresuspension by wave action.

A statistical assessment of predicted concentrations pro-vides an overview that would be useful for impact assess-ment. Figure 14 depicts contours of the maximum suspendedconcentrations predicted during the simulated period from 22January to 28 February 2005 and also contours of the timefor which a threshold of 100 mg/L was exceeded. Maximumconcentrations of up to 80 mg/L (southern dredge area), 190mg/L (northern dredge area), and 350 mg/L (near the plant)are predicted close to dredger and plant discharge sites. How-ever these elevated concentrations are short-lived. A concen-tration threshold of 100 mg/L was exceeded less than 1% ofthe time in a region of up to a few hundred metres from theplant discharge. This exceedance decreased to 0.05% withinan area extending �1 km alongshore, in a northwesterly di-rection, and up to �500 m offshore. Exceedance of the 100mg/L threshold concentration occurred at the northerndredge area only, for just over 2% of simulation time. Thearea within which this occurred had a diameter of �250 m.Examination of the less impacted southern dredging area in-dicated that the maximum predicted concentrations decayedto background levels (�5 mg/L) within 4 km north and 500m south of the southern dredging area.

The Fate of Fine Sediment: Deposition, Resuspension,and Transport

The amount and type of sediment recovered from sandtraps provide a rough assessment of the maximum potential



Figure 13. Predicted surface concentration (left) and bottom concentrations (right; note zoomed-in view) of suspended solids resulting from dredging atthe southern dredging site as well as plant tailings discharge.

deposition that could have occurred with the available sus-pended material. Three traps situated at Site A accumulatedan average vertical thickness of 4.1 mm of potential deposi-tion (standard deviation � 2.1 mm); 89% of the trapped ma-terial was found to be inorganic. At the control site trap, 6.2km to the south, an average of three traps indicated 6.4 mmof potential deposition on average (standard deviation � 1.5mm). Similar to Site A, most of the material (77%) was foundto be inorganic. Potential deposition at the dredging site wasless than at the control site, suggesting that the two sets oftraps were located in different depositional environments.Nevertheless, it can be inferred from the comparatively lowdeposition potential at Site A that the potential depositionfrom the dredging was orders of magnitude less than thatwhich could have occurred naturally.

While the potential for deposition does not seem severe,predictions suggest that the actual deposition was even lesssevere. Simulations indicate that deposition, particularly de-position in depths less than 40 m, is transient, lasting nomore than a few hours or days. Furthermore, an assessmentof maximum deposition thickness attained during the period22 January to 28 February 2005 indicated a maximum tran-

sient deposition thickness of only 0.05 mm (Figure 15 indi-cates contours of predicted deposition maxima).

Further insight into this transient, short-lived depositionof discharged material, followed by resuspension of the de-posited material, can be obtained through reference to thetime series of mid-depth concentrations measured at Site A(Figure 4). This time series indicates low concentrations afterdredging was completed at the southern dredging site. How-ever, a sudden increase in sediment concentrations was ob-served on 5 February 2005. A plot of the total bed shearstress (employing the standard DELFT3D model formula-tions) (WL � DELFT HYDRAULICS, 2003b), using the White-Colebrook friction coefficient for flow induced bed shear andthe TAMMINGA (1987) formulation for the wave friction fac-tor, indicates that the elevated concentrations were probablythe result of resuspension induced by increased bed shear.Assessment of the relative contributions of waves and wind-and tide-driven currents at Site A to the total shear stressindicates that the contribution of waves was consistently anorder of magnitude higher than that of the currents. The re-suspension event occurred in response to a rapid increase inwave height (of about 2 m in the offshore area). Such a con-

244 Smith et al.


Figure 14. Predicted distributions of maximum suspended concentrations (left) during the period from 22 January to 28 February 2005 and the timefor which a threshold of 100 mg/L was exceeded (right).

dition (of offshore significant wave height [Hmo] of just over3 m and a wave period [Tp] of about 12 s) would be expectedroughly 10% to 20% of the time (ROSSOUW, 2002). After thisinitial resuspension event, some response of sediment con-centration to subsequent increases in bed shear occurredeven though average wave conditions occurred at this time(with offshore significant wave height averaging about 2 mand wave period fluctuating from 7 to 14 s). However, sub-sequent resuspension of fine sediment was affected by re-duced sediment availability, since sediment was transportednorthward after the initial resuspension event. In addition,toward the end of the measurement period, the signal wasaffected by fouling of the turbidity sensor by marine growth,which causes a subtle increase in apparent concentrations.

These results indicate that fine sediment is highly mobilein a wave-dominated environment. It is therefore of interestto assess the input and loss of fine sediment from the modeldomain. Figure 16 illustrates a time series of the percentageof the total sediment discharged (at a particular time) thatremains within the model domain (i.e., an area of 109 km �56 km). The total daily sediment input from both dredgingoperations and plant tailings discharge is also indicated. An

initial buildup of sediment in the model (starting at 100%) ispredicted up to the time that sediment is transported to themodel boundaries. Judging by flows typical of the summerseason (e.g., Figure 6), this transport to the model boundarieswill take a matter of days. From model predictions that pro-vide insight into the direction of transport of fines (e.g.. Fig-ures 13 and 14), it may be inferred that the sediment is trans-ported predominantly to and ultimately beyond the north-western boundary of the model, situated over 60 km to thenorth-northwest.

The fraction of sediment remaining in the model seems torespond to both the quantity and the location of sedimentinput. The highest percentages of sediment retained in themodel occurred during the time of dredging. This percentagerapidly decreased when sediment input was solely throughplant tailings discharge into the intertidal zone from 4 Feb-ruary 2005. Even though sediment input was still high (be-tween 400 and 800 tons per day) after dredging ceased, thepercentage of sediment remaining soon declined to under10% of the sediment input. The difference in percentage sed-iment retained in the model between the two periods duringand after dredging is large. The lower percentage for the lat-



Figure 15. Predicted distribution of maximum deposition during the pe-riod 22 January to 28 February 2005.

Figure 16. Time series of sediment input (t/d) and predicted percentageof sediment remaining within the model domain.

ter period was probably due to the efficient transport mech-anism of wave-induced longshore currents, as the plant ma-terial was discharged directly onto the beach. On the otherhand, sediment discharged beyond the surf zone was sub-jected to relatively reduced flows and turbulence.

DISCUSSION

Measurements and predictions in this study generallymanifest an increase in concentrations with depth in the im-mediate vicinity of dredging activity. Further from the dredg-er, concentrations were found to be fairly uniform with depth,apart from a shear stress–induced layer of relatively higherconcentrations near the bed. While it is recognised that finesediment distributions are affected by dredging method, in-tensity, and duration, this trend of higher concentrations atdepth was nevertheless found to occur at other dredging op-erations (BLACK and PARRY, 1999; CARTER and COLES,1998). While settlement of sediment in the water column isan obvious reason for this trend, dilution of concentrations asa result of a vertical distribution of decreasing velocities mayalso play a role. However, close to the dredger, confused andrapidly varying vertical distributions of sediment concentra-tion are probably the result of dredger-induced flows and tur-bulence and initially rapid settling of discharged material,

particularly of the sand fraction. Such confused and even ‘‘in-verted’’ concentration profiles (i.e., with suspended solids con-centrations higher near the surface than at depth) were re-ported in MIKKELSON AND PEJRUP (2000) and are evident inthe data of BOHLEN et al. (1979).

This study predicted that maximum concentrations woulddecay to a value close to background natural concentrations�4 km north of and �500 m south of the southern dredgingarea. Recognising that the assessment of maximum predictedconcentrations was conservative, this distance of decay wasnevertheless somewhat higher than generally reported (HEA-LY et al., 1999; NEWELL et al., 1999; ONUSCHAK, 1982; RID-LEY THOMAS et al., 1998). This perceived retarded horizontaldecay of sediment concentrations, relative to these otherstudies reviewed, may have been due to the dominance ofswell waves at the study site (although it is acknowledgedthat the particular bed material, hydrodynamics, dredgingmethod, dredging duration, and dredging intensity will alsohave an influence). Such waves hinder deposition, therebykeeping sediment in suspension where it contributes to form-ing higher concentrations. Another study conducted at a morewave-sheltered bay on the southern Namibian coastline(SMITH et al., 2002) corroborates the tendency for limited de-position of discharged sediment in the bay under averagewave conditions.

Predicted persistence of the plume 3 h after dredging (Fig-ure 13) suggests a plume life span well in excess of 3 h beforedecay of concentrations to background levels would occur.This persistence of plumes is also probably a result of limiteddepositional conditions, which would promote longevity ofplumes.

A statistical assessment of predicted concentrations indi-cated that while instantaneous high maxima (highest being350 mg/L) occurred near the fine sediment sources, these el-evated concentrations were short-lived and were generally re-

246 Smith et al.


stricted to within a few hundred metres of the dischargesources.

A crude estimate of deposition ‘‘potential’’ from sedimenttraps indicated that the 4 to 6 mm of sediment depositionwas largely inorganic and would occur naturally. However,actual deposition is likely to be far less than indicated bythese measurements. Based on modelling, deposition indepths less than 40 m is transient, lasting no more than afew hours or days, with a maximum predicted depositionthickness of only 0.05 mm. The mobility of fine sediment issupported by the finding of minimal fine sediment at the sea-bed: results of a grain-size analysis of a sample taken fromthe surface of the seabed at the southern dredging site (25 mdepth) indicated that only 2% of the sample was composed ofsilt (sediment finer than 63 �m), 8% was gravel, and the re-mainder was sand. The prediction of meaningful depositiononly beyond 40 m depth (Figure 15) corresponds with obser-vations by geologists of the fate of fine material dischargedat the Orange River, which (after a stage of temporary de-position on a delta) occurs in depths greater than 40 m (ROG-ERS, 1977).

Measurements during the dredging exercise support theprediction of short-lived, transient deposition, whereby re-suspension of fine sediment is initiated after a very calm pe-riod following dredging at the southern site as a result ofrelatively frequently occurring swell wave conditions and isseen to persist under average wave conditions. As a result ofthis persistent mobility of fine sediment, the material is pre-dicted to be transported by wave- and wind-driven flows be-yond the model boundary situated over 60 km to the north-northwest. This result corroborates observations of HITCH-COCK and BELL (2004), who noted movement of sedimentfrom the source up to 35 km distance.

CONCLUSIONS

In this study, expected trends in concentrations of horizon-tal decay and higher values at depth were found. Somewhatexpected, confused, and at times inverted concentration pro-files were found near the dredger. The extent of dredgedplumes (i.e., where concentrations were very close to back-ground levels) was predicted to be some 4 km at most, whilea plume life span well in excess of 3 h, before decay of con-centrations to background levels, was indicated. This extentand longevity of plumes were attributed to the high mobilityof fine sediment in a wave-dominated environment resultingfrom elevated bed shear stress induced by swell wave action.As a result of the mobility of fine sediments, predictions in-dicate that they may be transported tens of kilometres withina matter of weeks.

ACKNOWLEDGMENTS

The authors would like to acknowledge the assistance andaccess to data provided by Namdeb Diamond Corporation(PTY) Ltd. Furthermore, the authors would like to acknowl-edge the efforts of the CSIR field team and Lucille Schone-gevel.

LITERATURE CITED

BACON, M.P., 1996. Evaluation of sediment traps with naturally oc-curring radionuclides. In: ITTEKKOT, V.; SCHAFER, P.; HONJO, S.,and DEPETRIS, P.J. (Eds.), SCOPE 57—Particle Flux in the Ocean.Chichester, UK: Wiley, pp. 85–90.

BLACK, K.P. and PARRY, G.D., 1999. Entrainment, dispersal and set-tlement of scallop dredge sediment plumes: field measurementsand numerical modelling. Canadian Journal of Fisheries & AquaticSciences, 56(12), 2271–2281.

BOHLEN, W.F.; CUNDY, D.F., and TRAMONTANO, J.M., 1979. Sus-pended material distributions in the wake of estuarine channeldredging operations. Estuarine and Coastal Marine Science, 9(6),699–711.

BOOIJ, N.; RIS, R.C., and HOLTHUIJSEN, L.H., 1999. A third gener-ation wave model for coastal regions. Part I: model description andvalidation. Journal of Geophysical Research, 104(C4), 7649–7666.

CARTER, R.A. and COLES, S., 1998. Saldanha Bay General CargoQuay Construction: Monitoring of Suspended Sediment Distribu-tions Generated by Dredging in Small Bay. Stellenbosch: CSIRReport ENV-S98100.

DOWNING, J.P.; STERNBERG, R.W., and LISTER, C.R.B., 1981. Newinstrumentation for the investigation of sediment suspension pro-cesses in the shallow marine environment. Marine Geology, 42,19–34.

GARDNER, W.D., 1980. Field assessment of sediment traps. Journalof Marine Research, 38(1), 41–52.

HEALY, T.; MEHTA, A.; RODRIGUEZ, H., and TIAN, F., 1999. Bypass-ing of dredged littoral muddy sediments using a thin layer dis-persal technique. Journal of Coastal Research, 15(4), 1119–1131.

HITCHCOCK, D.R. and BELL, S., 2004. Physical impacts of marineaggregate dredging on seabed resources in coastal deposits. Jour-nal of Coastal Research, 20(1), 101–114.

HSU, A., 1988. Coastal Meteorology. New York: Academic Press,260p.

JING, L. and RIDD, P.V., 1996. Wave-current bottom shear stressesand sediment resuspension in Cleveland Bay, Australia. CoastalEngineering, 29(1–2), 159–186.

KRONE, R.B., 1962. Flume Studies of the Transport of Sediment inEstuarial Shoaling Processes: Berkeley, California: University ofCalifornia, Hydraulic and Sanitary Engineering Laboratory.

LUDWIG, K.A. and HANES, D.M., 1990. A laboratory evaluation ofoptical backscatterance suspended solids sensors exposed to sand-mud mixtures. Marine Geology, 94, 173–179.

MADSEN, O.S.; POON, Y.K., and GRABER, H.C., 1988. Spectral waveattenuation by bottom friction: theory. Proceedings of the 21st In-ternational Conference on Coastal Engineering, ASCE, 1, 492–504.

MIKKELSEN, O.A. and PEJRUP, M., 2000. In situ particle size spectraand density of particle aggregates in a dredging plume. MarineGeology, 170, 443–459.

MILLIGAN, T.G., 1995. An examination of the settling behaviour ofa flocculated suspension. Netherlands Journal of Sea Research,33(2), 163–171.

NEWELL, R.C.; HITCHCOCK, D.R., and SEIDERER, L.J., 1999. Organicenrichment associated with outwash from marine aggregatesdredging: a probable explanation for surface sheens and enhancedbenthic production in the vicinity of dredging operations. MarinePollution Bulletin, 38(9), 809–818.

ONUSCHAK, E., JR., 1982. Distribution of suspended sediment in thePatuxent River, Maryland, during dredging operations for con-struction of a pipeline. Bulletin of the Association of EngineeringGeologists, 19(1), 25–34.

OWEN, M.W., 1971. The effect of turbulence on the settling velocitiesof silt flocs. Proceedings of the 14th IAHR Congress, Societe Hy-drotechnique de France, Paris, paper no. D4.

PARTHENIADES, E., 1962. A Study of Erosion and Deposition of Co-hesive Soils in Salt Water. Berkeley, California: University of Cal-ifornia.

RIDLEY THOMAS, W.N.; HALE, R.E., and LAND, J.M., 1998. Sus-pended sediment measurement in Hong Kong. Hydro Internation-al, 2(6), 61–63.

ROGERS, J., 1977. Sedimentation on the Continental Margin off the



Orange River and the Namib Desert. Cape Town, South Africa:University of Cape Town, PhD thesis.

ROSSOUW, M., 2002. Metocean Data Summary: Offshore of Oranje-mund. Stellenbosch: CSIR Report ENV-S-C 2002–028.

SMITH, G.G. and SOLTAU, C., 2002. Coastal Modelling Study at thePocket Beach Areas: Accretion, Safety, Environment. Stellen-bosch: CSIR Report ENV-S-C 2002-107.

SMITH, G.G.; MOCKE, G.P.; VAN BALLEGOOYEN, R.C., and SOLTAU,C., 2002. Consequences of sediment discharge from dune miningat Elizabeth Bay, Namibia. Journal of Coastal Research, 18(4),776–791.

SMITH, G.G.; WEITZ, N.; SOLTAU, C., and VILJOEN, A., 2006. As-sessment of the Cumulative Effects of Sediment Discharges fromOn-Shore and Near-Shore Diamond Mining Activities on theBCLME. Stellenbosch: CSIR Report CSIR/NRE/ECO/ER/2006/0093/C.

TAMMINGA, G.H., 1987. Influence of a constant south-westerly windon the erosion in Lake Ijssel. Unpublished report of the Hydraulicsand Discharge-Hydrology Department, Wageningen, January1987.

VAN RIJN, L.C., 1987. Mathematical Modelling of Morphological Pro-cesses in the Case of Suspended Sediment Transport. Delft Uni-versity of Technology, PhD thesis.

VAN RIJN, L.C., 1989. Handbook: Sediment Transport by Currentsand Waves. Delft Hydraulics Report H461.

VAN RIJN, L.C., 1993. Principles of Sediment Transport in Rivers,Estuaries and Coastal Seas. Amsterdam, The Netherlands: AquaPublications.

WL � DELFT HYDRAULICS, 2002. DELFT3D-WAVE User Manual Ver-sion 2.01. Delft, The Netherlands: WL � Delft Hydraulics.

WL � DELFT HYDRAULICS, 2003a. DELFT3D-FLOW User ManualVersion 3.10. Delft, The Netherlands: WL � Delft Hydraulics.

WL � DELFT HYDRAULICS, 2003b. DELFT3D-WAQ User Manual Ver-sion 4.00. Delft, The Netherlands: WL � Delft Hydraulics.

ZOUTENDYK, P., 1992. Turbid Water in the Elizabeth Bay Region. AReview of the Relevant Literature. Stellenbosch: CSIR ReportEMAS-I 92004.

ZOUTENDYK, P., 1995. Turbid Water Literature Review: A Supple-ment to the 1992 Elizabeth Bay Study. Stellenbosch: CSIR ReportEMAS-I 92004.

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